The present invention relates to a radio communication system using a portable mobile terminal. More specifically, the invention relates to a radio communication system for realizing a high transmission rate and a large quantity of information.
In recent years, with the development of mobile communication and the enhancement of the quantity of communication, it is expected that there will be a great demand for high-speed, large-capacity mobile radio communication systems using potable mobile terminals in the near future.
FIG. 1 illustrates a conventional mobile radio communication system. In this figure, a base station 1 forms an omni-directional beam 11, which can covers a given area, by means of a transmitting antenna, so as to allow communication to be established wherever a mobile terminal lies in the coverage area.
In addition, mobile terminals 2, 3 and 4 also form omni-divisional beams 12, 13 and 14 by means of antennas, so as to allow communication to be established regardless of the direction of the base station from each of the terminals.
With this construction, it is possible to realize a mobile communication system, which enable communication in any places. However, considering that information to be communicated will be large-capacity, high-capacity in the near future, it is desired to realize a radio communication link suitable for the high-speed, large-capacity information.
In order to obtain a high-gain transmitting-receiving antenna suitable for such a communication system, it is required to sharpen an antenna beam pattern. In this case, the base station and the terminals must have the constructions for forming shaper beams as shown in FIG. 2.
In FIG. 2, the base station 1 forms narrow beams 15, 16 and 17 which are capable of transmitting and receiving signals toward the terminals 2, 3 and 4. On the other hand, the terminals 2, 3 and 4 forms narrow beams 6, 7 and 8 which are capable of transmitting and receiving signals toward the base station 1.
For example, in a case where different frequency bands are allotted to the respective beams 15, 16 and 17 formed by the base station 1, the beams 15, 16 and 17 may be formed at the same time.
Alternatively, in a case where communication is established in a time-division system, the beams 15, 16 and 17 formed by the base station 1 may be switched every a predetermined period of time. In this way, it has been able to be technically practiced that the direction of a narrow beam is scanned by using a conventional phased array antenna.
FIG. 3 illustrates an example of a high-gain beam scanning antenna in a conventional radio communication system. In this figure, antenna elements 21, 22, 23 and 24 are connected to diplwxers or transmission-reception (TR) switches 25, 26, 27 and 28, respectively, so that the transmitting and receiving of a radio signal are separated.
The signals received by the respective antenna elements are supplied to corresponding phase shifters 32 via amplitude-variable amplifiers 30, and the amplitude and phase of the signals are weighted so that radio waves in a predetermined direction are received. The signals outputted from the respective phase shifters 32 are supplied to a receiver 37 via a synthesizer circuit 34.
With respect to transmission, the signals supplied from a transmitter 35 are distributed by means of a distributing circuit 33, and then the amplitude and phase of the signals are weighted by means of amplitude-variable amplifiers 29. Then, the signals are transmitted to the respective antenna elements via the TR switches 25, 26, 27 and 28, so that the synthesized directivity of the respective antenna elements forms a beam in a predetermined direction.
The phase shifters 31 and 32 and the amplifiers 29 and 31 are controlled by a controller 36, so that radio waves in a predetermined direction are transmitted and received by comparing received signal levels and so forth.
It is possible to perform the gain enhancement and the beam scanning according to the method as set forth above. However, considering that it is applied to an actual system, the construction, control and so forth of a radio transmitter-receiver are extremely complicated as described below.
Since the base station 1 communicates with terminals serving as the other parties in communication, it is required to discriminate the directions of the respective terminals to turn beams to the optimum directions for the respective terminals.
In addition, it must be assumed that the respective terminals move in mobile communication, so that it is required to cause the base station to independently follow the directions of all the terminals.
For the reasons as set forth above, in a case where a phase array antenna is used, independent feed circuits are required for each of beams, so that the construction of the base station is complicated. For example, in a case where communications between the base station and a plurality of terminals are established, a plurality of feed circuits for setting the individual amplitude and phase weight are required.
In addition, in a case where the direction of the beam (terminal) is switched by time base, a phase shifter and an amplifier must be switched at a high speed. In either cases, since a control system must be independently operated, the software therefor is complicated, and a great deal of labor is required in a processing system.
As mentioned above, although it is required to enhance the gains of antennas of a base station and terminals in accordance with the increase of the speed and capacity of communication which will be required in future, there is a problem in that the construction and control of an antenna feed circuit are extremely complicated.